Tuesday, February 26, 2019

Sean Carroll On Cosmology

  1. The Big Bang model is simply the idea that our universe expanded and cooled from a hot, dense, earlier state. We have overwhelming evidence that it is true.
  2. The Big Bang event is not a point in space, but a moment in time: a singularity of infinite density and curvature. It is completely hypothetical, and probably not even strictly true. (It’s a classical prediction, ignoring quantum mechanics.)
  3. People sometimes also use “the Big Bang” as shorthand for “the hot, dense state approximately 14 billion years ago.” I do that all the time. That’s fine, as long as it’s clear what you’re referring to.
  4. The Big Bang might have been the beginning of the universe. Or it might not have been; there could have been space and time before the Big Bang. We don’t really know.
  5. Even if the BB was the beginning, the universe didn’t “pop into existence.” You can’t “pop” before time itself exists. It’s better to simply say “the Big Bang was the first moment of time.” (If it was, which we don’t know for sure.)
  6. The Borde-Guth-Vilenkin theorem says that, under some assumptions, spacetime had a singularity in the past. But it only refers to classical spacetime, so says nothing definitive about the real world.
  7. The universe did not come into existence “because the quantum vacuum is unstable.” It’s not clear that this particular “Why?” question has any answer, but that’s not it.
  8. If the universe did have an earliest moment, it doesn’t violate conservation of energy. When you take gravity into account, the total energy of any closed universe is exactly zero.
  9. The energy of non-gravitational “stuff” (particles, fields, etc.) is not conserved as the universe expands. You can try to balance the books by including gravity, but it’s not straightforward.
  10. The universe isn’t expanding “into” anything, as far as we know. General relativity describes the intrinsic geometry of spacetime, which can get bigger without anything outside.
  11. Inflation, the idea that the universe underwent super-accelerated expansion at early times, may or may not be correct; we don’t know. I’d give it a 50% chance, lower than many cosmologists but higher than some.
  12. The early universe had a low entropy. It looks like a thermal gas, but that’s only high-entropy if we ignore gravity. A truly high-entropy Big Bang would have been extremely lumpy, not smooth.
  13. Dark matter exists. Anisotropies in the cosmic microwave background establish beyond reasonable doubt the existence of a gravitational pull in a direction other than where ordinary matter is located.
  14. We haven’t directly detected dark matter yet, but most of our efforts have been focused on Weakly Interacting Massive Particles. There are many other candidates we don’t yet have the technology to look for. Patience.
  15. Dark energy may not exist; it’s conceivable that the acceleration of the universe is caused by modified gravity instead. But the dark-energy idea is simpler and a more natural fit to the data.
  16. Dark energy is not a new force; it’s a new substance. The force causing the universe to accelerate is gravity.
  17. We have a perfectly good, and likely correct, idea of what dark energy might be: vacuum energy, a.k.a. the cosmological constant. An energy inherent in space itself. But we’re not sure.
  18. We don’t know why the vacuum energy is much smaller than naive estimates would predict. That’s a real puzzle.
  19. Neither dark matter nor dark energy are anything like the nineteenth-century idea of the aether.
From Sean Carroll's blog (a January 12, 2019 post).

He is mostly, but not entirely, correct. I have put what I agree with in bold, and what I think is wrong or overstated in strikeout, and that visually makes clear the extent to which I do and do not agree with his 19 statements about cosmology.

I agree with 1-12, 14, and 18.

I disagree with 13 ("Dark matter exists. Anisotropies in the cosmic microwave background establish beyond reasonable doubt the existence of a gravitational pull in a direction other than where ordinary matter is located."). Dark matter phenomena definitely exist and require "new physics" to explain, but the interpretation he gives to the CMB is more model dependent than he acknowledges. There are, however, at most, 50-50 odds that it is caused by dark matter particles rather than gravity modification or something similar. Also, many of the more viable dark matter particle theories require a fifth force or gravity modification in addition to dark matter particles. Personally, I think that an explanation predominantly from gravity modification (including subtle refinements of GR in either a classical or quantum gravity mode) is more likely than not to be correct.

The first sentence of 15 ("Dark energy may not exist; it’s conceivable that the acceleration of the universe is caused by modified gravity instead.") is true. The second ("But the dark-energy idea is simpler and a more natural fit to the data.") is not. 

I disagree with 16 ("Dark energy is not a new force; it’s a new substance. The force causing the universe to accelerate is gravity."). This is a possibility, but not anything approaching a certainty. Indeed 16 is internally inconsistent with 15.

The last sentence of 17 ("But we’re not sure.") is true. The first two sentences of 17 ("We have a perfectly good, and likely correct, idea of what dark energy might be: vacuum energy, a.k.a. the cosmological constant. An energy inherent in space itself.") are mostly true except for the "likely correct" part.

19 is mostly true, but "anything like" in 19 is susceptible to different interpretations and if you standard for similarity is low, it isn't true, so it slightly overstates this proposition. 


Graham Dungworth said...

Nineteen listed comments!Even the Good Lord presented but ten commandments and most of those were on adultery, the Big Bang.
There's overwhelming evidence for the BB, so why does he sit on the fence and claim only a 50/50 commitment? In 2011 90% of Cern scientists were so depressed they thought the chance of a Higgs particle discovery was minimal. A leaked email by the director stressed the importance that Susy particles should be emphasised to the world media. Some clever wag from the NSF tried to claim such things either exist or don't exist, hence the probability is 50/50. The scientific reply was that statistics don't kinda work that way.
So what was the Big Bang? Let us explain the situation ca. 1990 where Peebles, Weinberg and others left off.

A superhot dense phase >10^12K. Did it cool (evolve?) from an even higher temperature in the approach to the Planck temperature? At higher temperatures the number of particle species increases ( without limit?. ) Phase changes might limit the maximum temperature of the earliest instant to not much higher than 10^12 Kelvin.
This hot dense thermal "gas" is described as a thermal plasma, space filling fields of quantum condensates in true thermodynamic equilibrium, the condition of high entropy. With gravity however, the highest entropy state is one where particles clump together, black holes are the greatest digesters of entropy. Frank Wilczek didn't like the historical odium of that simple aether description and preferred instead, a condensed matter approach of fermi and boson quantum condensates, that during phase changes at constant entropy,condense out of the ............ or "aether" . Now quantum mechanics and quantum field theories didn't exist in the minds of late 19th scientists. Einstein's vacuum is free space, ie. free of particles, of Hertz's radiation it can be totally emptied. This spatial expansion doesn't affect the size of atoms, the Earth's size, stars and even galaxies over billions of years, the big rip happens later during a 10^100 year era. There's almost no dark energy at the commencement of the Big Bang; whereas matter density dilutes rapidly as size expands, dark energy density is constant; at least Guth replies that inflation is enormous and proceeds at constant density during the phase change.

Graham Dungworth said...

Sean used to claim that expansion contravened the 2nd law of thermodynamics which was true over the short term if the universe was closed but now the feeling is of continued accelerated expansion interpreted from SNe type supernova at high z (up to ~z=2). The size of the universe is given by size = 1 + z. The universe in ca. 40 gigayears in future will have doubled in size, the critical density will dilute 8 fold and the cmb temperature is predicted to be 2.725/2 Kelvin, assuming no phase change occurs. Dark Energy if it exists remains at constant density with size. By convention the cmb temperature extrapolates in the approach to 0 Kelvin(heat death). The hugely diluted universe is almost devoid of matter apart from black holes; their evaporation time is up to 10^100 years. It appears strange that we live in a time instant (0....1 to >10^100yr) when DE is within an order of magnitude of predicted DE, DM and normal matter densities and during this timeframe the cmb radiation has stretched and cooled from >>10^12K from z=1000 to now z=0 at 2.725 Kelvin! Hubble images range back to ~z=10. Galaxies we observe now, whose light was emitted ~10 billion years ago are now receeding at superluminal velocities; we know where they were then but not where they reside now. Weinberg refused to accept these greater than c velocities. There's no problem for some cosmologists because Einstein's free space was empty, with no energy content; these electromagnetic fields and quantum fields do contain energy.
There are reheating episodes in the Big Bang. Phase changes caused by the condensation of particles, and each particle in the Standard model with rest mass have empirically measured eV rest masses. They rain out of the OK -phew- quantum field fermi condensate at constant temperature and constant entropy. Steam undergoes a condensation to water droplets with a 1000/1 volume change. The free energy change is zero for these phase changes in that the numerosity of particle types govern the enthalpic internal energy change and the entropy change is given by Sean's motif S=k ln W . A matter and antimatter particle exist in two forms, exactly the same free eneries of formation but two particle forms exist and it's the enthalpic energies that drive the reaction. The same effect occurs in nature between left and right handed particles. Nature almost entirely adopts one form; evolution decides the survivors. Organic molecules come in several forms, say largely L. The D forms are toxic to many organisms.
The matter antimatter problems aren't in Sean's lists, nor is the horizon problem.
The list isn't exhaustive and many are interelated. Neutrinos have rest mass, what are their pair production temperatures?

andrew said...

"There's overwhelming evidence for the BB, so why does he sit on the fence and claim only a 50/50 commitment?"

He is 50-50 on cosmological inflation, not on the BB, and on that score he is right. The evidence for inflation is much more thin than the evidence for the BB. And, if it happened, it happened very early, very briefly and we have several hundreds of versions of it to distinguish from each other. The conventional timeline of the Universe doesn't really get to be solidly reliable until Big Bang nucleosynthesis (which itself has a minor kink related to lithium abundance to work out), which is still very early, but not that early.

He is also taking pains not to discount alternatives to the BB as the beginning of time, such as cyclical cosmologies, which provide alternatives to the outcomes that inflation is supposed to give rise to, which are favored in some quantum gravity theories. Cyclical cosmologies also provide easy solutions to the Baryon Asymmetry of the universe without requiring radically different CP violation new physics at high energies.

Graham Dungworth said...

In 2011 several ?Cern scientists asked. Are neutrino rest masses predicted in the particle Standard Model (SM)?
The answer eventually appeared. The SM predicts all particle masses are restmassless. It's the Higgs Fields, four of them that give mass to particles (maybe not the neutrinos) and in fact it's Sean who has presented the best explanation of the Higgs Boson or "My life as a Boson" blog personal Galaxy Zoo forum. At a 2011 conference comments I mentioned it was the only bet in town at the time ..the Higgs Boson should exist. 90% of Cerns particle physicists thought otherwise.
The Big Bang doesn't predict what the cmb temperature is. At our era in the 10^100 timeframe we measure it, and it happens to be 2.725K, an almost perfect thermal bath radiation, so we point to a theoretical cooling curve that starts at 10^12K or inflations 10^32K, that's some bang the BB. Peebles, Gamow etc previous predictions ignored the horizon, flatness matter/antimatter problems and came up with various coincidental temperatures ranging down from 20Kelvin to the heat death scenario. fortunately, we exist just at the era where all these numbers are measurable.
Things start to feel qite anthropic- particularly so for hoyle, Weinberg and lately Rees.
There are thousands of papers about production temperatures and phase changes of particles, atoms molecules. The last one in cosmology happened at z=~1000, almost 14 billion years ago. If we switch off the Higgs field everything everywhere is ripped apart and flies off at the speed of light. Well it hasn't happened since the field froze out before 10^-35 second, was still around at 10^-24second ( an eternity since 10^-35 instants of history) and even more so as it's still toggled on.
Might we not exist now in one of these longer lived phase changes. Is the evidence for this ancestral hot phase ( at least up to 10^12Kelvin thermal bath range) so overwhelming that we can claim with confidence that it's a 50/50 bet.

Graham Dungworth said...

Sean is infact a cosmologist and a great one at that, but there's something lacking in his makeup and judgement I am ill prepared or would wish to mention- faults we all suffer from. It's too harsh to write off his cosmological instincts.

Absolute neutrino rest masses we await; predictions we have and if correct Sean will be amongst the first to understand the impliclationsions to astronomy and a new cosmology.
The BB extrapolation is to a beginning 4 minutes later when the size of the universe is ca. 4 lyr ; our nearest star. The horizon and flatness issues were well known by Peebles, Weinberg.BB is untenable to them, to all of us. guth's inflaton theory is an attempt to solve the causality horizon problem.
The D and Li problems are not problems but arise from a top down approach. The universe starts hot , cools and there are bottleneck problems due to the instabilities of these nucleii. Hoyle knew them , he discovered their properties.

In a bottom up approach the universe starts cold with a phase transition that begins with collapse, a result of pair production in a thermal bath of photons and neutrinos at their pair production temperature. It's still happening and the next one is predicted to happen at a temperature of 0.89Kelvin some ~60gigayear hence. Te bottleneck doesn't happen as the system heats up. The same compositions will arise in a neutrino degeneracy of a "neutrino star". 0.00117ev is special since it predicts a minimum mass of 2*10^54kg for collapse (gravitational). Were an axion of mass some thousand fold smaller then the Universe would need to comprise a million fold greater. It's all very well to conceive of other universe where we can sweep our dust. I prefer to sweep dust under the carpet rather than chase a vacuum cleaner. Of course periodically I'll take the cleaner otherwise my wife complains.
I'm glad I did read Charles Darwin in the 70's but Alfred Russel Wallace in the early 90's was more momentous. Doesn't several hundred probable mistakes cry out for something more imaginative. Thousands came out about superluminary photons, probably caused by an over zealous cleaner.

Lavoisier, Gay Lussac, Carnot and Napoleon and Robert E Lee would understand.

Graham Dungworth said...

We are all flawed. That afternoon when I crunched out two pair production temperatures back in 2010, It was for the presumed normal hierachy first generation electron neutrinos 0.00038eV and 0.00117eV . What if the placnck charge was different for DM particles eg an alloH atom, that differed in that it's mass was 1/3 that of the electron , carge +- 1/3 and an allo proton, mass 1/3 and electric charge 1/3. Such a light allo element would have a bohr radius of 25 Angstrom in it's ground state. (Planck charge)^2 =e^2 *alpha. Fine structure constant 1/137 goes to 1/(137*9). The ionisation energy for H is 13.6 eV but for the allo case this scales as the inverse cube or 13.6/243 and so for allo allo brethren.

The lightest species or for neutrinos would have been the 0.000383 eV . I read Marni's post in Physics World The phase (2/9 +- Pi/12) appeared. In the next few days I was worried about why the temperature 0.89K didn't represent the cmb temperature. also, these were half masses in that nature violated parity and why weren't 4 species involved. We get the same thing in organic chemistry for instance.
Why do we see only the left handed neutrino, basically for the phase why don't we observe the phase expression +-2/9 +- Pi/12 Marni preferred the mirror expressions whereas I prefer the all iso nomenclauture of chemistry. It's not just that the cmb equates with the cmb temperature, why didn't it match the lightest state? viz a viz the lightest predicted supersym particles. Nature prefers the lowest energy state.
Optical active organics eg. simple amino acids can be treated as of tetrahedral symmetry, or chiral tetrahedral 24 order. In higher dimensions a 24 dimensional space affords the best packing density ie. lowest energy state, compared with all other numerosities.

Graham Dungworth said...

The idea of a phase change appeared, that the universal cmb might undergo a future phase change at 0.89 Kelvin. These contributors should be present in the cmb but subsumed within the 2.725Kelvin background with 1/89th of the energy density. This should show up in the current cmb profiles as a slight excess of temperature.
The extremely hot thermal origin is clumpy as a consequence of gravity. In the galaxies ectromagnetc discharges result in far greater disruptive effects than gravitational clumping yet this ancestral radiation after 14 billion years is an almost perfect thermal bath. A lengthy (Gigayears) phase change alternative drives the system towards equilibrium. Inflation and the BB , apart from e H combination at ca. 300,000 years at z=1000 all happen instantly on this scale of time. Overwhelming evidence is derived from a single measured point on a cooling curve. The pair production temperature of a massless neutrino was never considered.

I have been around a long time; it's a flawed world but it's still, still more a beautiful one, despite the horrors and calamities , poverty, suppression...we suffer and there is no alternative within reach . I am not really thick skinned but I have not let myself be subjected to the abuse and ridicule I suffered on my blog or what Marni has elsewhere. My blogs at the time represented the thoughts of a scientist, on a day to day basis, not polished, with the scaffolding left in place, the errors of thought still there , warts and all, no attempt was made to polish the text, absurd predictions etc. all there. I expressed at the beginning, to express these ideas, never to return and reread my old blogs. As I stated at the beginning never to return and reread them. I have stuck to this decision, only exceedingly rarely, to return and scan a page and recall that I couldn't remember writing on such a topic. That's why they are so numerous. In the early days one amateur scientist poster commented on the fact that it was incoherent rubbish, aka phlogiston theory. The following day or so Marni appeared and stated she didn't know enough about astronomy but if it was rubbish the particle physics part was coherent rubbish. All , never mine, these posts are now deleted!
The pool of candle wax condensate has consumed its flame.
Goodnight folks 11.58 pm here.
Best regards Andrew
I'm not a robot ?? Graham